224 SCIENCE PROGRESS. 



cortical tissue. When this is the case in the adult, it is still 

 often possible to make the separation in the young stem. 



Naming the central cylinder a stele, we call all stems 

 with a single cylinder monostelic. 



Stems in which we cannot make the separation in any 

 part, and which are therefore not strictly monostelic, yet 

 conform more or less to the monostelic structure in other 

 respects, and are no doubt usually derived in descent from 

 the monostelic type. 



Most Ferns and Selaginellas, and two genera of 

 Phanerogams, while showing a monostelic structure in 

 their hypocotyls, possess in their later formed stems more 

 than one cylinder, each comparable in structure to the single 

 stele of the hypocotyl. Such stems are known as polystelic. 

 The steles of a polystelic stem may, however, take on the 

 most various forms, and lose all the characters of the 

 original cylinder ; several may even coalesce to form a 

 structure indistinguishable from a single stele. As this, or 

 indeed the converse case of a non-stelar vascular strand 

 assuming the characters of a stele, may have happened in 

 descent without leaving any traces of the transformation, we 

 are not justified in asserting the homology of all steles or 

 denying homology between steles and non-stelar vascular 

 strands. Nevertheless the stele is undoubtedly a real and re- 

 latively stable type in the arrangement of vascular tissue, and 

 hence the name represents a real morphological conception. 



The vascular tissue of a leaf is arranged in one or more 

 strands, each of which, bilaterally rather than radially 

 symmetrical, is called a schistostele or meristele, representing, 

 as it does, a part only of the stem cylinder. The meristele 

 of a petiole may, however, simulate a stele. In most poly- 

 stelic stems one or more of the stem steles directly enters 

 the petiole, and the branches maintain more or less of the 

 stelar character till near their endings in the lamina, where 

 they become indistinguishable from collateral bundles. 



We are probably justified in supposing the monostelic 

 type to be primitive in vascular plants, and we may assume 

 the original stele to have been relatively simple. To the 

 increase in bulk of the stem and correlated increasing de- 



